The present invention relates in general to the field of utility meters. More particularly, the present invention relates to systems and methods for temperature-dependent and frequency-dependent phase, shift compensation of low permeability current sensors in electronic energy meters.
Programmable electronic energy meters are rapidly replacing electro-mechanical meters due to the enhanced functionality achieved using programmable logic integrated into solid-state electronic meters. Some of these meters can be used to meter various different electrical services without hardware modification. For example, meters having a voltage operating range between about 98 Vrms and about 526 Vrms are capable of operation with either 120 V or 480 V services. U.S. Pat. No. 5,457,621, dated Oct. 10, 1995, entitled SWITCHING POWER SUPPLY HAVING VOLTAGE BLOCKING CLAMP, assigned to ABB Automation Inc. discloses examples of such meters. In addition, some meters are constructed for use with any 3-wire or any 4-wire service, also disclosed in U.S. Pat. No. 5,457,621.
Electronic energy meters are instruments that measure the flow of energy. Electronic energy meters typically do this by sensing the current and voltage. The power is derived from the sensed currents and voltages, and energy is defined as the measurement of power over time.
Voltage and current signals are primarily sinusoidal. Voltage and current sensors are used in a meter to convert the primary signals to a signal that can be processed. One type of current sensor commonly used in electronic meters is a current transformer. In an ideal current transformer the secondary current is equal to the primary current divided by the turns ratio. In practice, current transformers are non-ideal, having losses in the burden, the copper wire in the windings, and the core itself. These characteristics result in amplitude and phase deviations as compared to an ideal current transformer.
The current transformer""s phase shift is predominately determined by the inductance, the winding resistance, and the burden resistance. The current transformer essentially behaves as a high pass filter with the inductance and the sum of the winding and burden resistances setting the break frequency. In order to reduce this phase shift error, electronic energy meters typically use core materials having a very high relative permeability to obtain a high inductance. It is not uncommon for a core""s relative permeability to be as high as 100,000 in order to achieve phase shifts of less than 0.1 degrees.
In some markets, it is desired for meters in direct-connected applications to be accurate even in the presence of significant half-wave rectified currents. An example of this can be found in the IEC-1036 requirements. As a half-wave rectified waveform has significant DC content, it is necessary for current sensors in such meters to be sufficiently immune to DC in the primary current. High permeability cores become saturated quickly in the presence of DC current and hence have limited application with this requirement.
For current transformers, immunity from DC current can be improved by increasing core area, by selecting alternative core materials that have a higher saturation level, and by lowering the relative permeability of the core material. In general, increasing the core geometry is limited due to cost and space requirements. Examples of alternative core materials are nanocrystalline and amorphous materials. These materials have recently become economically feasible and reliable. Although such materials improve the DC immunity it is still necessary to lower the overall relative permeability to provide an appropriate solution.
This DC immunity comes at a cost, however. As the permeability and inductance of the current sensors are reduced, the phase shift error is greater. With phase shifts greater than about 0.5 degrees, changes in the phase shift with operating conditions can no longer be ignored. The current transformer""s inductance is a function of the line frequency and the winding resistance is a function of temperature (as a result of the copper wire). Thus, the phase shift is a function of temperature and frequency, and because the phase shift in low permeability materials is larger, they are more sensitive to temperature and frequency. Thus, a need exists to compensate for the frequency and temperature induced errors in the phase and amplitude output of the current sensors in an electronic energy meter.
The present invention is directed to a system and method for compensating for temperature-induced and/or line frequency-induced changes in the phase shift of the current sensors in an energy meter. To compensate for temperature-induced phase shift, a temperature reading from a temperature sensor within the energy meter is obtained. The temperature reading is converted to a digital signal. The digital signal is then converted to a degrees of phase shift value. A processor in the meter adjusts its output based on the degrees of phase shift value.
To compensate for line frequency-induced phase shift, a line frequency of the signals is obtained. The line frequency is converted to an engineering units value. The engineering units value is then converted to a degrees of phase shift value. A processor in the meter adjusts its output based on the degrees of phase shift value.
To compensate for both temperature-induced phase shift and line frequency-induced phase shift, the respective degrees of phase shift values are combined to obtain a total degrees of phase shift value. The processor then adjusts its output based on the total degrees of phase shift value.
According to aspects of the invention, converting a digital signal or an engineering units value to a degrees of phase shift value comprises solving an associated linear equation for phase shift based on temperature or line frequency. The linear equation is determined by an approximation of the theoretical and experimental data.
According to further aspects of the invention, the output of the processor is delayed by an amount equal to the degrees of phase shift value, or by a time shift determined based on the degrees of phase shift value.